1. Field of the Invention
This invention relates in general to a paper feeding device and a sheet carrying device wherein the sheet loaded on a carrier plate can be detected and the sheet can be fed towards a predetermined position.
2. Description of Related Art
A conventional paper feeding device comprises a paper-feeding roller, a paper-detecting member and a plate-shaped paper guide. The paper-feeding roller feeds a sheet through a tray loaded with sheets such as documents to be read or stationery etc. The paper-detecting member is arranged at an upstream side in a carrying direction of the paper-feeding roller. The plate-shaped paper guide is set between the paper-detecting member and the paper-feeding roller. Only a small gap exists between the paper guide and the sheet loaded on the tray (Japan Laid-open Publication no. He7-330182).
In an automatic document-feeding device installed in a facsimile or a copier machine, a mechanical type (a filler type) document-detecting sensor is set in a document set portion. The length of the filler depends on the size of the paper-feeding path (the gap which the paper passes through). If the paper-feeding path is large, the filler has to be longer, while if the paper-feeding path is small, the filler has to be shorter. That is because the filler crosses (overlaps) an upper guide plate and a lower guide plate.
Here, referring to FIG. 8A, if the gap between the upper guide plate 11 also serving as a cover and the lower guide plate 12 is large, the thin document 13 will bend towards a home position of the filler of the document-detecting sensor 2 due to a recovery force. As a result, the thin document 13 cannot be detected by the document-detecting sensor 2. This drawback can be overcome by overlapping the filler of the document-detecting sensor 2 and the upper guide plate 11 as shown in FIG. 8B.
However, for loading a plurality of paper on the document setting unit, the paper feed path must be set to be wide enough. Here, referring to FIG. 9A, if there is a gap between the filler of the document-detecting sensor 2 and the upper guide plate 11, if the total height of loaded document 13 is increased, the upmost document 13 may not be detected by the document-detecting sensor 2. On the contrary, referring to FIG. 9B, if the gap between the filler of the document-detecting sensor 2 and the upper guide plate 11 is small, as the upper guide plate 11 is lowered down, the number of sheets of the document 13 for loading must be reduced. Moreover, though it has been considered to extend the filler of document-detecting sensor 2, this will obstruct the operation of the filler so that the layout of the filler cannot be done. For example, if the filler installation space is narrow, it is difficult to extend the filler.
Regarding the technique disclosed in Japan Laid-open publication no. He7-330182 as shown in FIG. 10, a sheet-shaped paper guide (MYLAR®) 14a is set at the upstream side in the paper-feeding direction of the filler of the document-detecting sensor 2. The MYLAR® 14a actuates the document 13 towards the detecting side of the filler of the document-detecting sensor 2 according to the number of sheets of the document 13.
However, in the conventional paper feeding device, if there is a gap between the sheet-shaped ® 14a and the document 13 (for example, fewer number of sheets of the thin document 13 is set), the thin document 13 carried through the gap still cannot be detected by the document-detecting sensor 2. This may delay the thin document 13 from moving to the side of the feeding roller. Moreover, under this condition, the front end of the document 13 will move up and down due to the vibration when the device is driven. The operation for detecting the document 13 is not stable. Additionally, in the structure for holding a pick up roller through a magnetic clutch, one sheet of document 13 is picked up to feed and the magnetic clutch is switched off when the rear end of the document 13 is pulled by the pick up roller. Then the front end of the document 13 to be picked up is free to move up and down. Moreover, to prevent sheets of document 13 from being stacked to be fed, the second document 13 is pushed back by the operation of a one-way clutch. In the structure (separation mechanism) for preventing a double feed, under the condition that the front end of the document 13 is free, the document 13 will be pushed back so much that the detecting operation of the document-detecting sensor 2 is not stable.
Furthermore, during opening or closing the upper guide plate 11 (serving as a cover) of a paper-feeding unit which comprises the pick up roller and the feed roller or during pulling out the document 13 (from the carry position), the front end of the sheet-shaped MYLAR® 14a bends backward to the upstream side in the carrying direction. This situation may obstruct the setting of thin paper or the setting of a fewer-sheet document 13.